HD001123 and HD001124 are companion projects that together investigate auditory communication in primates and other vertebrates. The overall goal of these studies is to provide a comprehensive understanding of primate auditory communication in terms of development, neural mechanisms, endocrine factors, and social context. Two non-human primates, the squirrel monkey and common marmoset, are the main subjects of study, with additional data collected from other species where appropriate. Prior work in this project using brain lesioning methodology has shown that production of sounds that are the functional and acoustic equivalents of cry sounds in human infants are mediated by limbic cortex located along the anterior midline of the frontal lobe of the cerebral cortex, and that single neural elements in the auditory cortex (superior temporal gyrus) are particularly responsive to subtle differences in the acoustic structure of species-specific vocalizations, suggesting an important role in mediating individual differences (vocal signatures). A new initiative in the laboratory employs immunocytochemistry to identify the neurons activated in crying infants and in adults hearing infant cries. A study of c-fos expression in infant marmoset monkey brains was conducted. The paradigm involved separating individuals for 30 minutes, recording the cry sounds made during this separation, returning them to their home cage for 1 hour, then euthanizing them and processing their fixed brains for Fos using immunocytochemistry. Of particular note is that regions found to be strongly labeled in adult monkeys were also strongly labeled in infants. In particular, the anterior cingulate gyrus, midline cortex in the frontal lobe, had numerous labelled cells even at 1 month of age. These results indicate that c-fos immunocytochemistry is a method that can demonstrate functional involvement of brain regions also active in adults using the same behavioral paradigm. A brain atlas developed in this laboratory in collaboration with investigators in the NINDS Laboratory of Functional and Metabolic Imaging provided the means to identify the specific structures containing large numbers of Fos. A study begun last year used similar methods to identify the neurons activated in adult listeners upon hearing infant cries. Analysis of 3 male brains indicates that structures associated with the limbic system of MacLean, including the anterior cingulate gyrus, gyrus rectus, septum, amygdala and hippocampus, all have significant numbers of Fos-labeled cells. In addition, the superior temporal gyrus, where the auditory cortex is located, is richly labeled. Using MRI images from a female marmoset, structures identified and labeled in the lab's histological brain atlas were also identified in the MRI images (closely matched to the plates from the histological atlas), making it possible to more accurately identify structures in the marmoset brain in functional MRI studies. An MTA with Monash University will permit investigators there to use the MRI images from this atlas to develop new anatomical tools for viewing and identifying structures in the marmoset brain. A new initiative this year has added identification of fiber tracts in the brain to a revised atlas. This is being done using myelin-stained sections matched to the cell structure sections, and DTI images matched to the myelin-stained sections. Identification of fiber tracts in the myelin-stained and DTI images will provide, for the first time, a comprehensive picture of fiber tracts in the marmoset brain. A new study this year (in progress) using c-fos immunocytochemistry is examining areas in the brains of squirrel monkeys who are high- or low-vocalizers in the context of social separation. The pathways in the squirrel monkey brain for vocal production are well known, hence this will provide the first confirmatory information about vocal structures as revealed by the c-fos expression technique.